Pebble bed reactor

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Gas-cooled high-temperature reactor with a core of spherical fuel and
moderator (graphite) elements. The decommissioned nuclear power plants
AVR in Jülich and THTR-300 in Uentrop had a pebble bed reactor.
The THTR-300 contained about 600,000 spherical fuel and moderator elements.
The fuel elements consist of a kernel of U-235 and thorium surrounded
by a graphite matrix of a diameter of 6cm.

Schematic representation of the spherical fuel element
of a pebble bed reactor